1. Field of the Invention
The invention relates to testing of electrical systems and, more particularly, to apparatus for testing of electrical distribution devices, such as, for example, a receptacle outlet or a wiring device.
2. Background Information
A glowing contact is a high resistance connection, which can form at the interface of a copper wire and a screw terminal, for example, of a receptacle. The resulting temperature rise at this connection point can melt the wire's insulation and damage the receptacle. It is desirable to be able to detect this condition and interrupt the current before the glowing contact fault progresses to a hazardous condition.
The hazard associated with aluminum wiring has been known and understood for thirty years. The connection of an aluminum wire conductor to the terminal of a wiring device is unstable, since the aluminum, over time, tends to flow, thus, making the aluminum wire-to-terminal a high resistance connection. The resulting I2R heating causes local heating that can melt the wire's insulation and the receptacle. It was believed that simply returning to copper wire would resolve this problem. Unfortunately, this is not true. Furthermore, most people, outside of the standards and wiring device industry, are unaware of the glowing contact problem. Also, the lack of wide spread public knowledge of the glowing contact problem may follow from the fact that there has been no known solution to this problem.
It is very easy to create a high resistance or glowing contact at a receptacle terminal using copper wire. See, for example, Sletbak, J., et al., “Glowing Contact Areas in Loose Copper Wire Connections,” IEEE, 1991, pp. 244-48.
The hazards associated with glowing contacts, including contacts made with all combinations of copper, brass and iron are known. See Yasuaki Hagimoto, “Japanese Reports on Electrical Fire Causes,” 1996, 12 pp.
In a similar manner that aluminum oxide creates the aluminum wire problem, the culprit associated with a glowing contact is copper oxide. There are two recognized mechanisms for creating a high resistance copper oxide contact: arcing; and fretting. The arcing mechanism involves, for example, a loose receptacle screw terminal and slight movement of the wire while it is carrying a current. Every time the electrical connection is broken, a single electrical arc discharge can occur. Fretting involves wire-screw connection motion without breaking the connection.
FIG. 1 shows the voltage across the terminal-to-wire connection in the upper trace (about 170 V peak) and the current through that connection in the lower trace (about 15 A peak) for different intervals of an electrical connection being broken while carrying current. This pair of voltage and current traces is broken into three intervals I,II,III. The first interval I shows normal operation in which there is negligible voltage across the terminal-to-wire connection, which has a relatively low resistance, with an alternating current flowing through that connection. During the second interval II, there is a significant terminal to wire-screw connection voltage due to a single arcing (negative) half cycle event. Hence, there is a corresponding reduction in the magnitude of the alternating current flowing through that connection. Finally, during the third interval III, the terminal-to-wire connection becomes an open circuit and the voltage across the terminal-to-wire connection is the line voltage. As a result of the open circuit, there is essentially no current flowing through that connection.
While there is essentially very little power dissipated in the terminal-to-wire connection during the first and third intervals I,III, relatively significant arcing and power dissipation occurs in the second interval II. To the extent that the second interval II may become relatively periodic or persistent, then oxidation can occur at the copper wire-screw interface where the half cycle arcing has occurred with each breaking of the wire-screw connection. This copper oxide layer at the wire-screw interface can also occur due to the mechanism of fretting or a rubbing action with no arcing.
By Paschen's laws, it is not possible to create a sustained copper-to-copper through air arc discharge in a 120 VRMS circuit with a resistive load. An arc is formed when the contact breaks, although it extinguishes at the first zero current crossing, since the voltage is too small for a “re-strike”. This is sometimes called a “spark” rather than an “arc”. There can be a spark whenever an electrical contact is broken due to local heating at the break point. Hence, an inductive load is needed for sustained cycle-to-cycle arcing in most 120 VRMS residential wiring, other than a 240 VRMS circuit. Otherwise, with a resistive load, a peak voltage of about 300 volts is needed in order to create a sustained arcing event as compared to an available peak value of about 170 volts for a 120 VRMS circuit.
Each single arc discharge forms a small amount of copper oxide (Cu2O) at the terminal-to-copper wire interface. With repeated discharges, the amount of the copper oxide increases over time. Copper oxide has a number of characteristics which, when combined, creates a hazard. First, the interface can be mechanically strong. Hence, once the terminal-to-copper wire connection is made through the copper oxide, the connection may become permanent. Second, copper oxide is a semiconductor that has a very high negative resistance-versus-temperature characteristic between about 180° C. and about 250° C. Over this temperature range, the resistance decreases as much as five orders of magnitude. As the connection heats, the current tends to concentrate into a relatively narrow region, thereby resulting in a very high current density and temperature. For example, a spot temperature of about ˜1200° C. may result. The wire and screw temperatures near the connection point can exceed 300° C., which temperature is hot enough to melt, for example, the wire's insulation and the receptacle's plastic housing, but not the copper oxide.
During a glowing contact fault in a receptacle, the copper wire reaches a glowing temperature value at which time the wire looks like an electric heater coil. First, the wire's insulation melts at the terminal and, then, slowly progresses away from the terminal toward other wires in the receptacle's outlet box. This can result in either an arcing fault or a ground fault if the bare glowing wire contacts another conductor. Second, the heat resulting from the glowing contact fault flows into the receptacle and causes the plastic housing of the receptacle to melt. As the plastic melts, the receptacle loses its mechanical integrity and, thus, the electrical isolation between conductors is compromised. This may ultimately lead to either a line-to-ground fault or a neutral-to-ground fault. In the event that the upstream protective device (e.g., a circuit breaker) does not respond, then the plastic could ignite.
FIG. 2 shows a receptacle with a flowing contact at the line terminal T resulting in the insulation melting away from the line conductor C in the area of that line terminal.
Once a glowing contact is formed, the current during the formation of the glowing contact and the subsequent current flowing through the glowing contact is typically normal, since the voltage drop across a glowing contact is typically about 2 VAC as shown in FIG. 3. The existence of a glowing contact, therefore, is not reliably detectable by a conventional upstream current protective device (e.g., a conventional circuit breaker or fuse). However, significant damage may result to both the wire's insulation and the receptacle. On the other hand, if an upstream circuit breaker with both arc and ground fault protection is employed, then that circuit breaker will respond to arcing or a ground fault resulting from insulation damage caused by a glowing contact and will eventually trip in order to de-energize the branch circuit, thereby protecting the damaged wire and/or receptacle.
High resistance connections, such as cause glowing contacts, are most typically “behind the wall” and, thus, are hidden. Hence, there is the need for the detection of such hidden problems. Accordingly, there is room for improvement in apparatus for testing electrical distribution devices, such as receptacle outlets and wiring devices.